This invention relates to a technology which is effective in controlling a resonating wavelength and a resonating output in a laser irradiating device.
A laser beam has some features of a coherent high purity of wavelength and a high output or the like and it is highly expected as a light source capable of irradiating an intensified beam. In recent years, there has been developed a light source device using such a laser beam. Its typical one is a narrow band type excimer laser that is studied as a light source for lithography.
In order to get a laser beam of which band is narrowed, it is necessary to provide a configuration having a wavelength selection device such as a grating, a prism, a birefringent filter and an etalon etc. utilized as a laser resonator.
For a laser medium having a laser gain at a wide band such as an excimer laser or a dye laser, it has been applied to insert one or a plurality of etalons into a laser resonator in order to make a narrow band region.
The etalon is a wavelength selecting element in which multireflection of light and interference phenomenon of light generated between the two reflection films in horizontal orientation having a high degree of flatness are utilized, wherein the first etalon may act as a rough adjusting component for a narrow band and the second etalon may act as one for fine adjusting component. That is, the original laser resonation wavelength is roughly narrowed by the first etalon, for example, and this is further narrowed by the second etalon up to a desired band width and then it is outputted.
In FIG. 6, is indicated a wavelength dependability of transmitting light in a typical etalon. A characteristic of the etalon is designated by a spacing of the transmitting bands and a transmitting band width. The spacing of the transmitting band is defined as a free spectral range (hereinafter called as an FSR) and this is dependent upon a spacing between the reflection films. The transmitting band width is defined as a width of a half (1/2) of a peak height in the transmitting band range. Then, a ratio between an FSR and the transmitting band width is called as a fineness (i.e. a fineness=FSR/a transmitting band width) and this is determined by a reflection rate, a degree of parallel and a degree of flatness of the reflection film. The narrow band of the laser beam can be realized by utilizing the two etalons of which FSR and fineness are properly selected.
In case where KrF excimer laser is narrowed for its band width by utilizing the etalon having the aforesaid configuration, the band width is narrowed by about 1/10 of the original laser resonating band region by the first etalon and then the width is narrowed by 1/10 by the second etalon.
In case of the normal laser resonating device, an optospectrum measuring device and an output measuring device or the like are additionally arranged for the aforesaid arrangement and the resonating wavelength and the resonating output were stabilized through a feed-back control on the basis of these measured data. However, it was normally applied to vary a power supply voltage of a laser power supply in order to make a forced control over the resonating output.
To the contrary, in case where the power supply voltage was increased in order to increase the resonating output of laser through the aforesaid technology, in particular, in case of applying gas laser such as KrF excimer laser, deterioration of gas was remarkable and then a reduction of laser output was frequently generated.
In order to accommodate for the reduction of the output of the laser, it was necessary to increase the power supply voltage gradually and finally there was a possibility that the voltage reaches its upper limit value.
In addition, in case of the control over the resonating output under a control of the power supply voltage as described above, it was hard to make a fine adjustment of the output.
In order to get such a band width as one required for the power supply for a lithography, it is necessary to make a narrow transmitting band width of etalon. By this fact, it is necessarily required to increase a fineness, resulting in that a reflection film having a high reflection rate is applied. The reflection film is normally got by a coating of a multi-layer film of dielectric material and in order to increase a reflection rate, it is necessary to increase the number of coating layers. Due to this fact, there arise some problems in which a manufacturing step of the etalon is complicated and its reliability in operation and price are not assured. In addition, if the etalon having a high reflection rate is inserted into the laser resonator, its loss caused by the reflection is increased and the output of the resonated laser beam is decreased.
In turn, under the same transmitting band width, the lower FSR, the lower fineness, resulting in that the reduction of output can be prevented. However, a mere reduction of only FSR causes the light to be leaked out of the transmitting band width adjacent to the central frequency of transmitting beam (that is defined a side band), resulting in producing an inconvenient status.
It is an object of the present invention to enable a control over a laser resonating output to be attained without using any control over a power supply voltage, thereby to enable a prevention of deterioration of gas and fine adjustment of output to be realized and at the same time to realize the narrow band formation without increasing a fineness of etalon used by making a proper design of the etalon.